The present invention relates generally to turbochargers, and relates more particularly to exhaust gas-driven turbochargers having an axially sliding piston for varying the size of a nozzle opening leading into the turbine wheel of the turbine so as to regulate flow through the turbine.
Regulation of the exhaust gas flow through the turbine of an exhaust gas-driven turbocharger provides known operational advantages in terms of improved ability to control the amount of boost delivered by the turbocharger to the associated internal combustion engine. The regulation of exhaust gas flow is accomplished by incorporating variable geometry into the nozzle that leads into the turbine wheel. By varying the size of the nozzle flow area, the flow into the turbine wheel can be regulated, thereby regulating the overall boost provided by the turbocharger's compressor.
Variable-geometry nozzles for turbochargers generally fall into two main categories: variable-vane nozzles, and sliding-piston nozzles. Vanes are often included in the turbine nozzle for directing the exhaust gas into the turbine in an advantageous direction. Typically a row of circumferentially spaced vanes extend axially across the nozzle. Exhaust gas from a chamber surrounding the turbine wheel flows generally radially inwardly through passages between the vanes, and the vanes turn the flow to direct the flow in a desired direction into the turbine wheel. In a variable-vane nozzle, the vanes are rotatable about their axes to vary the angle at which the vanes are set, thereby varying the flow area of the passages between the vanes.
In the sliding-piston type of nozzle, the nozzle may also include vanes, but the vanes are fixed in position. Variation of the nozzle flow area is accomplished by an axially sliding piston that slides in a bore in the turbine housing. The piston is tubular and is located just radially inwardly of the nozzle. Axial movement of the piston is effective to vary the axial extent of the nozzle opening leading into the turbine wheel. When vanes are included in the nozzle, the piston can slide adjacent to radially inner (i.e., trailing) edges of the vanes; alternatively, the piston and vanes can overlap in the radial direction and the piston can include slots for receiving at least a portion of the vanes as the piston is slid axially to adjust the nozzle opening.
The sliding-piston type of variable nozzle offers the advantage of being mechanically simpler than the variable-vane nozzle. Nevertheless, other drawbacks have generally been associated with sliding-piston type variable nozzles. The piston must be somewhat smaller in diameter than the inner diameter of the turbine housing bore to ensure that the piston can freely slide without binding. As a result, a potential leakage pathway exists through the inevitable gap between the piston and bore. Leakage of exhaust gas through this pathway reduces turbine performance.
Furthermore, dimensional changes in the turbine housing and/or piston as a result of thermal expansion and contraction can lead to growth of the gap and hence increased leakage. Typically the piston is of a different material from that of the turbine housing, and the two materials have different coefficients of thermal expansion. As a result, it is generally necessary to design the piston-to-housing clearance on the high side at low temperatures to avoid binding of the piston at high temperatures, or vice versa, depending on the relative coefficients. Accordingly, during some operating conditions the gap between the piston and housing is relatively large and leads to high gas leakage, which is harmful to turbocharger performance.